The present invention regards a catalytic composition comprising gallium, at least one element chosen in the group of the lanthanides, and a zeolite of MFI, MEL or MFI/MEL structure, the crystal lattice of which is made up of silicon oxide and at least one metal oxide chosen from among aluminum oxide, boron oxide and gallium oxide. Preferably, in the catalytic compositions of the present invention a zeolite is used belonging to the MFI family characterized by crystallites which for at least 90% have a diameter smaller than 500 Å and which can form agglomerates of submicron dimensions characterized by possessing at least 30% of the extrazeolitic porosity in the region of the mesopores.
In addition, the catalytic compositions of the present invention can contain rhenium.
These catalytic compositions are useful in processes of aromatization of aliphatic hydrocarbons having from 3 to 6 carbon atoms.
The reaction of aromatization of paraffins and light olefins (C2-C5) to yield mixtures of benzene, toluene, ethylbenzene and xylenes (BTEX) has for many years been a subject of study. In 1973, the use was described of zeolites having an MFI structure (ZSM-5, ZSM-11, ZSM-21) for the aromatization of light hydrocarbons (both saturated and unsaturated) resulting from cracking, and from the production of coker gasoline or pyrolysis gasoline (U.S. Pat. Nos. 3,756,942 and 3,845,150).
U.S. Pat. Nos. 4,175,057 and 4,180,689 describe the reaction of aromatization of propane and butane in the presence of a catalyst with a base of gallium and an MFI zeolite. These patents were followed by numerous others regarding various modifications of this process involving modifications of the catalyst (U.S. Pat. No. 4,795,844), of the throughput (EP 252705, EP 050021 and U.S. Pat. No. 4,350,835) and of the system of introduction of gallium (EP 120018 and EP 184927). In particular, EP 252705 describes a process for producing aromatic compounds from feedstock containing C2-C12 aliphatic hydrocarbons, using a catalyst comprising a zeolite, having a constraint index from 1 to 12, a preferably very high silica/alumina ratio, and from 0.5 to 10% of gallium. Possibly, other elements chosen from among the metals belonging to the Groups I-VIII may be present.
It has moreover been found that the addition of platinum and palladium to the Ga and MFI zeolite-based catalyst determines an improvement in the aromatic-compound selectivity and reduces the formation of coke on the catalyst (U.S. Pat. No. 4,407,728 and EP 215579, 216491, 224162, 228267). The presence of these metals increases, however, the formation of methane and ethane deriving from cracking. Subsequently, it was found that the introduction of rhenium, in the presence of platinum or palladium, determines a further improvement in the aromatic-compound selectivity, but also in this case there is an increase in the amount of C1-C2 light paraffins among the products (U.S. Pat. No. 4,766,265). Catalytic compositions containing copper, or chromium, and an MFI zeolite determine the formation of smaller amounts of methane, but the aromatic-compound selectivity remains smaller than the one obtained with catalytic compositions containing gallium and an MFI zeolite (P. Meriaudeau et al., Zeolites: Facts, Figures, Future, 1423-1429, 1989; E. S. Shapiro et al., International Symposium on Zeolites as Catalysts, Sorbents and Detergent Builders, Wurzburg (RFA), p. 73, 1988).
Also described are catalysts containing an MFI zeolite, a noble metal of the Pt family, a metal chosen from among Sn, Ge, In and Pb, and an alkaline and/or alkaline earth component (EP 474536). This catalytic system involves an improvement in the aromatic-compound selectivity as compared to the foregoing materials.
All the catalytic systems described above are characterized by a very short life, since, an account of the high temperatures necessary for the reaction of aromatization of olefins and light paraffins, there is an important phenomenon of fouling and formation of coke inside the pores of the catalyst. This phenomenon is linked essentially to phenomena of cracking and/or of polycondensation of the compounds present in the reaction environment.